Studies on the genetics and pathogenesis of bluetongue virus (BTV) will continue to make use of wild-type (WT) and vaccine (V) strains of three BTV serotypes (10, 11, and 17) present in the United States. These studies will make use of a genetic approach to study basic features of BTV biology and pathogenesis and will focus on four main areas: (1) A test of our hypothesis to explain the genetic stability of the polyvalent, attenuated BTV vaccine. Well defined reassortants will be constructed from virulent and attenuated parental strains and used to test our hypothesis that genome segment 2 mutations are responsible for the attenuated phenotype of each of the component serotypes in the polyvalent vaccine. Our hypothesis allows prediction of the virulence/attenuation phenotype of each reassortant when they are used to infect a sensitive host-sheep. (2) An analysis of the reassortment of BTV genome segments during mixed infection of the various host species. These analyses will be performed to test our hypothesis, based on preliminary results, that because the arthropod vector (Culicoides variipennis) of BTV yields reassortants following mixed infection at a frequency nearly an order of magnitude higher than does a vertebrate host (sheep), the vector species may be a more important host than the vertebrate host for the evolution of BTV. (3) We will analyze the genetics of pathogenesis of BTV infection. These studies will include identification of the BTV viral attachment protein, and an analysis of the determinants of tropism in a cardiotropic mutant of BTV. Attenuated vaccine strains will be analyzed to determine if they have altered viral attachment determinants and altered tropism. (4) We will analyze mutational and structural rearrangements in the BTV genome. The mutations that occur during the long-term infection of some hosts (cattle) will be analyzed to determine if the evolution that occurs during the long infection is directed or random. A structural analysis of BTV genome segments containing gross rearrangements will be performed to determine the mechanism by which the rearrangements occur. The rearrangements to be examined have been seen to occur during the attenuation of BTV in cultured cells and during infection of the vector. These studies have as their unifying theme a focus on questions related to control of arthropod-borne viral disease by vaccination. They will provide significant new information on the genetics and pathogenesis of BTV infection. In addition, they will provide information important in devising vaccine strategies for important human diseases caused by viruses with segmented genomes or having arthropod transmission. The answers to the questions addressed here are critical to any strategy making use of polyvalent attenuated vaccines for control of viruses with segmented genomes.